Fedora core 5 + ISPConfig

Fedora core 5 + ISPConfig are installed on the server.
My server’s name is ns.nestor.org.gr .
Using ISPConfig I create a new site …..
Hostname: www
Domain:nestor.org.gr
IP Address: 195.251.5.3

All the users that use outlook to get their e-mails before the new installation of the server used to give at outlook settings as SMTP and POP3 servers the name “mail.nestor.org.gr” .
Now this doesn’t work and a solution is to give the IP address.
How I can fix this problem and make the mail server able to bee seen as “mail.nestor.org.gr” ?